Spinal alpha(2)-adrenergic and muscarinic receptors and the NO release cascade mediate supraspinally produced effectiveness of gabapentin at decreasing mechanical hypersensitivity in mice after partial nerve injury.

After partial nerve injury, the central analgesic effect of systemically administered gabapentin is mediated by both supraspinal and spinal actions. We further evaluate the mechanisms related to the supraspinally mediated analgesic actions of gabapentin involving the descending noradrenergic system. Intracerebroventricularly (i.c.v.) administered gabapentin (100 microg) decreased thermal and mechanical hypersensitivity in a murine chronic pain model that was prepared by partial ligation of the sciatic nerve. These effects were abolished by intrathecal (i.t.) injection of either yohimbine (3 microg) or idazoxan (3 microg), alpha(2)-adrenergic receptor antagonists. Pretreatment with atropine (0.3 mg kg(-1), i.p. or 0.1 microg, i.t.), a muscarinic receptor antagonist, completely suppressed the effect of i.c.v.-injected gabapentin on mechanical hypersensitivity, whereas its effect on thermal hypersensitivity remained unchanged. Similar effects were obtained with pirenzepine (0.1 microg, i.t.), a selective M(1)-muscarinic receptor antagonist, but not with methoctramine (0.1 and 0.3 microg, i.t.), a selective M(2)-muscarinic receptor antagonist. The cholinesterase inhibitor neostigmine (0.3 ng, i.t.) potentiated only the analgesic effect of i.c.v. gabapentin on mechanical hypersensitivity, confirming spinal acetylcholine release downstream of the supraspinal action of gabapentin. Moreover, the effect of i.c.v. gabapentin on mechanical but not thermal hypersensitivity was reduced by i.t. injection of L-NAME (3 microg) or L-NMMA (10 microg), both of which are nitric oxide (NO) synthase inhibitors. Systemically administered naloxone (10 mg kg(-1), i.p.), an opioid receptor antagonist, failed to suppress the analgesic actions of i.c.v. gabapentin, indicating that opioid receptors are not involved in activation of the descending noradrenergic system by gabapentin. Thus, the supraspinally mediated effect of gabapentin on mechanical hypersensitivity involves activation of spinal alpha(2)-adrenergic receptors followed by muscarinic receptors (most likely M(1)) and the NO cascade. In contrast, the effect of supraspinal gabapentin on thermal hypersensitivity is independent of the spinal cholinergic-NO system.